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Pyroxene fractionation in ferrobasalts from the Galapagos Spreading Centre

Published online by Cambridge University Press:  05 July 2018

I. D. Muir  and
D. A. Mattey
I. D. Muir
Affiliation:
Department of Earth Sciences, University of Cambridge
D. A. Mattey
Affiliation:
Department of Earth Sciences, University of Cambridge
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Abstract

The significance of metastable and equilibrium pyroxene fractionation trends in tholeiitic magmas is discussed, and the development of sector zoning and skeletal growth are considered in relation to Nakamura's (1973) hypothesis of protosites on growing clinopyroxene crystal surfaces. At the Galapagos Spreading Centre (GSC) basalts and ferrobasalts investigated behave paradoxically in that the slower-cooled basalts follow the quench trend while the faster-cooled ferrobasalts define a much closer approach to the equilibrium trend.

It is concluded that under metastable conditions fractionation trends in Ca-rich pyroxenes may be strongly influenced by textural features such as cotectic crystallization of plagioclase and the onset of liquid immiscibility, the latter leading to the development of strongly Fe-enriched ferroaugites. Even under plutonic conditions metastable crystallization can develop and severely reduce the pyroxene miscibility gap. A model for metastable crystallization is presented. These considerations are then addressed to the remarkable correspondence of the Skaergaard and Thingmuli pyroxene fractionation trends.

Type
Research Article
Information
Mineralogical Magazine , Volume 45 , Issue 337 , 1982 , pp. 193 - 200
Copyright
Copyright © The Mineralogical Society of Great Britain and Ireland 1982

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Footnotes

*

Department of Earth Sciences, University of Cambridge, Contribution No. 195.

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